Extremists and conspiracy theorists only see what they want to see.  Weird!
While it's an example of confirmation bias, it doesn't explain why they are like that.  When a person seems normal in most areas and goes full-on raging wackadoodle in a particular subject, it piques my interest.   It's not mental illness and is most likely related to trauma. 
Fredo and Perry are examples of mental deficiency, but 
@domer76 strikes me as normal in every regard except when discussing religion.  Hence, trauma is the mostly likely causal factor.